The present invention relates to a resin pulley which is used as an idler pulley for guiding a belt in a belt transmission system between a crankshaft and an auxiliary in, for example, a motor vehicle engine.
In a motor vehicle, an idler pulley for guiding a belt is provided in a belt transmission system which transmits rotational power of an engine to an auxiliary such as an alternator. In recent years, a resin pulley is used for such an idler pulley for reduction in weight and costs (for example, refer to JP-A-7-63249).
FIG. 9A is a side view showing an upper half of a conventional resin pulley, and FIG. 9B is a sectional view taken along the line C-C and as viewed in a direction indicated by arrows C in FIG. 9A. A conventional resin pulley 110 includes an outer cylindrical portion 120 round an outer circumferential surface of which a transmission belt is wound, an inner cylindrical portion which is disposed radially inwards of and concentrically with the outer cylindrical portion and on an inner circumferential surface of which a rolling bearing 111 is mounted, a circular disk portion 123 which connects together the inner cylindrical portion 121 and the outer cylindrical portion 120 at an axial center between the inner cylindrical portion 121 and the outer cylindrical portion 120, a plurality of ribs 122 which connect together the inner cylindrical portion 121 and the outer cylindrical portion 120 on both axial sides of the circular disk portion 123. The outer cylindrical portion 120, the inner cylindrical portion 121, the circular disk portion 123 and the ribs 122 are molded integrally with a rolling bearing 11 inserted therein through injection molding.
In the resin pulley 110 shown in FIGS. 9A and 9B, since the ribs 122 are formed radially in such a manner as to be directed in radial directions, the ribs 22 are in such a state that they project vertically relative to the outer cylindrical portion 120. Because of this, unevenness tends to be easily produced on the outer circumferential surface of the outer cylindrical portion 120 between portions where the ribs 22 are formed and portions spanning between the ribs during injection molding, this deteriorating the roundness of the resin pulley. Since the belt is wound round the outer circumferential surface of the outer cylindrical portion 120, the deterioration in roundness of the outer circumferential surface constitutes a cause for generation of noise and/or reduction in durability of the belt.
In addition, the deterioration in roundness of the resin pulley tends to take place easily when the resin pulley is molded from a thermal plastic resin such as nylon, and there occurs almost no such deterioration in roundness when the resin pulley is molded from a thermosetting resin such as a phenol resin having high dimensional stability. However, since thermosetting resins have a drawback that they are difficult to be recycled, thermoplastic resins are preferably used to mold resin pulleys while increasing roundness from the viewpoint of conservation of environment.
On the other hand, in the resin pulley 110 shown in FIGS. 9A and 9B, since the ribs 122 and the circular disk portion 123 are in such a state that they project vertically relative to the outer cylindrical portion 120, when a load is exerted on the outer cylindrical portion 120 from the belt, stress tends to be concentrated on the outer cylindrical portion 120 in the vicinity of joining portions where the ribs 122 and the circular disk portion 123 are joined to the outer cylindrical portion 120. In addition, since impact and vibration from the belt are transmitted from the outer cylindrical portion 120 to the inner cylindrical portion 121 via the ribs 122 substantially without being dampened, the rolling bearing 111 will have to be subjected to large impact and vibration. Because of this, there tends to be generated damage or wear such as a white layer flaking in the rolling bearing 111, leading to the possibility that the life of the rolling bearing 111 is reduced.
In addition, since the circular disk portion 123 is formed on the resin pulley 110 shown in FIGS. 9A and 9B, an axial air flow moving towards the resin pulley 110 is cut off by the circular disk portion 123 so formed. Because of this, the heat dissipating properties of the resin pulley are reduced, and hence, grease poured into the rolling bearing 111 tends to be thermally affected to thereby be deteriorated in quality or performance.